In the production of kraft pulp, black liquor is a common waste liquor. The black liquor must be passed to a recovery stage (that is have the chemicals therefrom recovered), or disposed of, and to facilitate either of these options, it is desirable that the black liquor have a low viscosity. Typical black liquor has a high viscosity, however. It is known per se that the viscosity of black liquor will decrease if it is heated under certain circumstances, however this fact has not heretofore resulted in practical application in a pulp mill for the continuous production of kraft pulp.
According to the present invention, the black liquor removed from the processing of comminuted cellulosic fibrous material into kraft paper pulp is acted upon in such a way that it has a lower viscosity than is conventional, and additionally it adds sulphur compounds to the cellulose material so that the pulp produced will have a lower kappa number than if treated conventionally (e.g. a kappa number of lower than 20). By heating the black liquor above cooking temperature (e.g. to about 200 degrees C.) after it is withdrawn from a continuous digester, and then recirculating it to a point prior to where the material is fed to a continuous digester, sulphur therefrom mixes with the material. The black liquor is then withdrawn from contact with the material, and fed to a recovery or disposal stage, the black liquor having a lower viscosity than when just withdrawn from the digester.
According to one aspect of the present invention, a method of producing kraft pulp from comminuted cellulosic fibrous material, using a continuous digester, is provided. The method comprises the steps of continuously: (a) steaming the comminuted cellulosic fibrous material; (b) adding white liquor to the steamed material; (c) cooking the material, with white liquor, in the continuous digester at a cooking temperature; (d) extracting black liquor from the continuous digester; (e) heating the extracted black liquor above the cooking temperature; (f) adding the heated black liquor from step (e) to the steamed material between steps (a) and (b); and (g) withdrawing the black liquor from the material between steps (f) and (b). Step (c) is typically practiced at a temperature of approximately 170 degrees C., in which case step (e) is practiced to heat the black liquor to a temperature of about 200 degrees C. (e.g. about 20-40 degrees C. over the cooking temperature).
The method of the present invention also may comprise the further step (h), between steps (e) and (f), of maintaining the material at above cooking temperature a time sufficient to insure significant reduction of the viscosity thereof. Step (h) may be practiced in a reaction vessel, and alkali may be added to the black liquor prior to, or simultaneously with, heating thereof. The black liquor withdrawn in step (g) is passed to a recovery or disposal stage, and steps (a)-(g) are practiced so that the black liquor at this stage has a significantly lower viscosity than the black liquor withdrawn in step (d). The white liquor may also be passed in heat exchange relationship with recirculating liquid prior to a high pressure feeder, to cool the recirculating liquid and heat the white liquor.
The invention also contemplates an apparatus for treating comminuted cellulosic fibrous material to produce kraft pulp, comprising a steaming vessel; a generally upright treatment vessel operatively connected to the steaming vessel; an upright continuous digester, operatively connected to the treatment vessel, the digester having a material inlet at the top thereof, and a material outlet at the bottom thereof; withdrawal screen means located at an intermediate portion of the digester, between the inlet and outlet thereof, for withdrawing black liquor from the digester into a withdrawal conduit; heating means for heating the withdrawn black liquor in the withdrawal means; recirculating means for recirculating the heated withdrawn black liquor to the treatment vessel at a first point thereof; and withdrawal means for withdrawing black liquor from the treatment vessel at a second point thereof, vertically spaced from the first point.
The apparatus may further comprise means for adding white liquor to the treatment vessel at a third point, closer to the digester than the second point. There also may be means for adding alkali to the digester black liquor withdrawal conduit before the heating means, and a reaction vessel for maintaining the heated black liquor at elevated temperature, between the heating means and the recirculating means. Also, the treatment vessel may be an impregnation vessel, comprising a first interior, vessel with an inlet and separator at its bottom and an open top, and a second exterior vessel in communication with the open top of the first vessel and having an outlet at its bottom.
The invention further contemplates another method for treating comminuted cellulosic fibrous material to produce kraft pulp, comprising the steps of: Steaming the material in a steaming vessel. Treating the material in a generally upright treatment vessel operatively connected to the steaming vessel. Cooking the material, at a cooking temperature, to produce kraft pulp in an upright continuous digester, operatively connected to the treatment vessel, the digester having a material inlet at the top thereof, and a material outlet at the bottom thereof. Withdrawing black liquor from the digester into a withdrawal conduit through withdrawal screen means located at an intermediate portion of the digester, between the inlet and outlet thereof. Heating the withdrawn black liquor in the withdrawal means above the cooking temperature. Recirculating the heated withdrawn black liquor to the treatment vessel at a first point thereof; and withdrawing black liquor from the treatment vessel at a second point thereof, vertically spaced from the first point.
It is the primary object of the present invention to provide a method of producing kraft pulp with low kappa number, while producing low viscosity black liquor for subsequent recovery or disposal. This and other objects of the invention will become clear from a detailed inspection of the invention, and from the appended claims.